Semiconductor packages are increasingly moving toward miniaturization, lighter and multi-functions for mobile application, which requires highly integrated multi-stack package. To meet the industrial demand, the package and silicon chip become thinner, and ultra-thin packages will show serious reliability problems such as warpage, crack and other failures. These problems are mainly caused by the mismatch of various package materials and geometric dimensions. In this study we perform the numerical analysis of the warpage deformation and thermal stress of 4-layer stacked FBGA package after EMC molding and reflow process, respectively. After EMC molding and reflow process, the package exhibits the different warpage characteristics due to the temperature-dependent material properties. Key material properties which affect the warpage of package are investigated such as the elastic moduli and CTEs of EMC and PCB. It is found that CTE of EMC material is the dominant factor which controls the warpage. The results of RSM optimization of the material properties demonstrate that warpage can be reduced by $28{mu}m$. As the silicon die becomes thinner, the maximum stress of each die is increased. In particular, the stress of the top die is substantially increased at the outer edge of the die. This stress concentration will lead to the failure of the package. Therefore, proper selection of package material and structural design are essential for the ultra-thin die packages.